Abstract Electronic cigarettes (e-cigs), were introduced to the US market only 10 years ago, and are markedly different from traditional cigarettes. Traditional cigarettes contain tobacco and are smoked through combustion, thereby exposing the smoker and those nearby (secondhand smoke; SHS) to thousands of potentially dangerous tobacco and combustion-related agents. In contrast, E-cigs are ?vaped? rather than smoked, and the resultant aerosols produced contain a far smaller number of potentially toxic chemicals, such as nicotine and flavorings, as well as byproducts caused by the heating of propylene glycol, and glycerin. Thus, they are widely believed and marketed as a safer alternative to cigarette smoking, yet the actual dangers posed by the use of these e- cigarettes remains largely unexplored. Many see the widespread adoption of this alternative nicotine delivery system as holding great promise in helping people quit or reduce cigarette use, and thus are seen as having profoundly positive effects as harm reduction agents. Others fear that their use may lead adolescents (the group most likely to begin using cigarettes) to transition from their use to the use of cigarettes. To date, missing from this debate is the potential biologic harm of mainstream and secondhand e-cig aerosols in homes where these agents are used, and how such exposure compares to mainstream and SHS from cigarettes. Our overall hypothesis is that e-cig vaping in homes leads to exposure to aerosols containing a number of pollutants and that this exposure is associated with decrements in cardiopulmonary function and increases in biomarkers for risk of future pulmonary and cardiovascular disease both in the e-cig vaper and non-smoking/non-vaping children and adults in such homes. Findings from this study will have very significant research and public health implications by enhancing understanding of the potential dangers of e-cigs, which are rapidly increasing in use and which many believe is a safe alternative to cigarette smoking and an aid to helping tobacco smoking cessation.